This invention relates to an actuator arrangement for use in aerospace applications. In particular, it relates to an actuator arrangement suitable for use in driving, for example, a variable area fan nozzle (VAFN) cowl, thrust reverser cowl or flight control surface such as a wing mounted flap or slat for movement between retracted and extended or stowed and deployed positions. For convenience, the description herein relates primarily to the use of the actuator arrangement in controlling the movement of and position occupied by a VAFN cowl, but it will be appreciated that the invention is not restricted in this regard.
Typically, hydraulically driven actuators have been used to drive VAFN cowls between their retracted and extended positions. Recently, there has been a move towards driving VAFN cowls, and other movable aircraft components, using electrically powered actuators. By way of example, a variable speed electric motor may be used to drive a linearly extendible actuator to control movement of the VAFN cowl. Appropriate position sensors may be used to monitor the position of the actuator and/or cowl, and the position information used in controlling the operation of the electric motor so as to slow the rate of movement of the cowl as it approaches its fully extended and fully retracted positions. Stops are typically provided in order to limit the range of movement of the cowl, preventing movement of the cowl beyond a predetermined position in the event of, for example, a powered runaway condition in which the motor fails to slow the cowl as the end of the normal range of movement is approached. Under normal operation, movement is arrested shortly before or as the stops engage, and the loads borne by the stops are thus relatively low other than in exceptional circumstances.
The provision of a variable speed motor and associated control circuits can result in the actuator arrangement being relatively large and heavy. As only limited space is available to accommodate the various aircraft components, and as the use of heavy components on an aircraft will negatively impact upon its efficiency, it is desirable to avoid the use of such components.
One possibility which has been considered is to replace the variable speed motor and associated control circuits with a more simple, fixed speed motor controlled in such a manner as to drive the actuator to a point at which the stops are engaged. Once this point is reached, the motor will stall, and stalling of the motor may be sensed, providing an indication that the cowl is fully extended or fully retracted, depending upon the direction in which it was being moved. Once a motor stall has been sensed the motor may be de-energized to terminate drive to the actuator. The use of a fixed speed motor operating in an open loop fashion allows significant savings to be made in terms of size, weight and cost, and simplifies the system considerably.
Whilst such an arrangement may achieve savings in relation to the motor and associated control circuit, it may result in the need to provide larger, more robust stops as the stops will be required to prevent over extension or retraction of the actuator during every operating cycle thereof, rather than serving to prevent over extension or retraction only in the event of a powered runaway condition or other exceptional circumstances. As the stops must be capable of repeatedly arresting actuator movement, with the actuator being driven at high speed immediately prior to engagement of the stops, it will be appreciated that large, heavy, robust stops are required in order to ensure reliable operation. The provision of such large stops is undesirable.
It is an object of the invention to provide an actuator arrangement in which at least some of the disadvantages associated with the arrangements outlined above are overcome or of reduced effect.